When a rocket motor undergoes slow heating (i.e., slow cook-off), such as in a fire onboard a naval vessel or the like, the propellant is gradually energized from the outer case wall inward. Slow cook-off is defined as ambient air temperature rising at about 6 degrees F. per hour. Heating proceeds until the propellant near the case wall auto-ignites. Problematically, when the propellant near the case wall auto-ignites, a high-pressure gas bubble generated inside the case wall collapses the propellant grain, blocking the associated nozzle and any vents, causing the pressure within the rocket motor to increase, and resulting in the explosion of the rocket, which is likely still housed in the related storage/launch assembly.
Therefore, it is desirable that a TRIM be provided that preemptively ignites a rocket motor undergoing slow heating before the propellant can auto-ignite, resulting in the propellant burning as designed, along the proper burning interfaces, instead of exploding. This mode of operation keeps the propellant grain intact and the venting channels open. If the rocket chamber is robust enough to contain the increase in pressure that results from burning the propellant at elevated temperature, then no further changes to conventional rocket systems would be necessary. If the rocket chamber is not robust enough to contain the increase in pressure that results from burning the propellant at elevated temperature, then larger venting channels or a stronger rocket chamber would be necessary. In any case, a controlled burn of a stored rocket is desirable as compared to an uncontrolled explosion of a stored rocket.
Several TRIMs based on a Nickel titanium (NiTiNOL) shape memory alloy (SMA) have been built to demonstrate SMA technology in the past. Such SMAs recover their unstrained shape when heated above a transition temperature. All have been unsuitable for application in a service environment. The first holds a percussion primer in line with an igniter passage, which is problematic in all but testing applications because in an operational application the accidental ignition of the percussion primer would fire the rocket motor. The second holds a percussion primer out of line from an igniter passage, but does not isolate the percussion primer from the environment. This structure, and related situation, could lead to injury if a person were handling the rocket. Thus, neither demonstration TRIM is acceptable for use in operational ordnance. A refined solution is still required.